The CD8+ cytotoxic T lymphocyte (CTL) response has an important protective role in the immunopathogenesis of HIV-1 infection, and is a key focus of vaccine development. However, the factors that determine the effectiveness of CTL against HIV-1 remain poorly understood. Unlike current vaccines that mimic naturally protective immunity, a successful HIV-1 vaccine will need to elicit CTL responses that are superior to those in natural infection. We hypothesize that HIV-1 mutational escape from CTL, particular in early infection, initiates a cascade of events that dooms the HIV-1-specific CTL response to long term failure. Early escape puts the CTL response a step behind HIV-1, allowing early events including irreversible CD4+ T cell depletion and original antigenic sin. Thus, addressing basic questions about the function and interaction of HIV-1-specific CTL with infected cells is key to the rationale design of a vaccine that will target CTL responses to avoid the naturally occurring mechanisms of failure. Our prior research has established in vitro models using HIV-1-infected cells to test questions that are difficult to address with correlative studies in vivo, e.g. that early protein epitopes can be recognized earlier in the life cycle than late protein epitopes. For this third funding period of the grant, we propose to continue examining factors affecting the interaction of CTL and HIV-1 using these systems, as well as to extend our studies to in vivo correlations. Specifically, we aim: 1. To assess for differential fitness costs involved in early and late CTL escape after HIV-1 infection;2. To determine whether epitope avidity thresholds change over the course of infection or correlate to susceptibility to CTL escape;3. To investigate the role of TCR breadth in preventing HIV-1 escape from CTL. PUBLIC HEALTH RELEVANCE: Historically successful vaccines have worked against pathogens where survivors of infection develop protective immunity by mimicking this natural immunity;for HIV-1, infected persons generally fail to contain the virus, and thus a vaccine that mimics natural immune responses against HIV-1 will likely fail. The goal of this project is to continue our ongoing studies to explore mechanisms of failure of the cellular immune response to contain HIV-1 infection. Understanding these mechanisms may be important for designing vaccine strategies that promote more effective responses than occur in nature.